In the medical field, there exists a number of applications requiring the warming of blood and intravenous (IV) fluids. For example, in connection with cardiac surgery during extracorporeal blood circulation, the patient is first cooled in order to slow metabolism and thereafter there is a requirement that the circulating blood be warmed. Another application is the warming of blood or intravenous fluids in a trauma situation. For example, heated IV fluids are useful in hypothermic patients and in trauma patients requiring massive IV resuscitation.
One common technique for warming blood is to pass the blood through coils immersed in a warm water bath. Microwave heating has also been employed in connection with the warming of blood and IV fluids. An example of an in-line microwave warmer for blood and IV fluids is described in U.S. Pat. No. 5,073,167. That apparatus is advantageous because it monitors the liquid temperature radiometrically and controls the heating power level based on the temperature measurements taken so that close control is maintained over the temperature of the fluid being warmed. Typically, the fluid exiting the warmer should have a temperature close to normal body temperature, i.e., 37.degree. C.
However, the physical sizes of those prior warmers necessitates that they be supported on an IV pole relatively remote from the patient. Resultantly, the fluid exiting those conventional blood warmers immediately begins to cool to room temperature and this cooling increases as the rate of infusion slows and the length of the patient IV line increases. In many cases, the warm IV fluid may sit in the patient line, slowly dripping its way to the patient's IV site. The constant exposure of the fluid to room temperature, e.g., 20.degree. C., steadily reduces the temperature of the fluid. In some cases, the problem is exacerbated because the volume of fluid infused at a low flow rate may be quite large relative to the size of the patient, particularly, in pediatric and neonate applications, causing a significant decrease in the patient's temperature.
In actuality, in the year 1989, blood transfusions of 1 to 2 units at flow rates less than 25 ml./min. accounted for more than 60% of the 3.2 million U.S. patients who received a total of 12.1 million units of red blood cells. Fluid exiting a typical warmer with a temperature of 37.degree. C. at a flow rate of 100 ml./hour cooled to 24.4.degree. C. after traveling through a typical length (105 cm) of patient IV tubing; see Faries G., Johnston C., Pruitt K. M., Plouff R. T.: "Temperature Relationship To Distance And Flow Rate Of Warmed IV Fluid", ANN EMERG MED 20: 1189-1200, 1991.
Relatively recently, a warmer has been developed to address low flow rate warming. The design uses a water jacket to surround the patient IV line. The water jacket, in turn, connects to a pole-mounted water bath maintained at a temperature of about 40.degree. C. by a 300 watt heater and circulating pump. Thus, the device surrounds the patient line with a layer of circulating warm water all the way to the patient and thus substantially eliminates patient line cool down. However, that device is disadvantaged because the water jacketed tubing is relatively large in diameter and inflexible making it difficult to attach to the catheter which introduces the infusate into the patient and adding mechanical stress to the catheter connection at the infusion site. Because of this, it has been found necessary to add a short length of non-insulating IV tubing between the jacketed tubing and the catheter thereby further fostering the patient line cool down problem.
Also, there is always a risk associated with the use of warm water for heating because of the danger of infection. Bacteria grows rapidly in warm water, requiring great care to prevent contact between the warm water and the tubing interconnections during a warming procedure. Also, to ensure sterility, the warm water bath must be emptied and cleaned regularly to avoid possible contamination.
Another problem with prior warmers generally is that they have a relatively large residual or priming volume, e.g., 20 ml or more. This is an important consideration for at least two reasons. First, a blood warmer may be on an infant or small child during extracorporal blood circulation. A typical infant may have a total blood volume of only 40-50 ml. This means that during the blood warming process, a large percentage of the patient's blood is outside the body at any given time which could cause patient trauma. Secondly, sometimes the infusate being administered includes very expensive drugs. If the infusion line includes a warmer with a high residual volume, the large amount of drug that remains in the line after the patient receives his prescribed dosage must be thrown away at a loss or be recovered which is expensive. Another possibility is to infuse the patient with saline interrupted by a dose or window of drug. However, this requires close monitoring of the infusion process which is labor intensive.
Finally, the conventional blood warmers of which we are aware are relatively bulky and heavy devices which have a relatively large footprint. Therefore, they are difficult to move around and difficult to locate close to a patient to avoid the long tubing lines to and from the patient.